Having served as the backbone of Shell's North Sea assets for more than 35 years, the Brent oil and gas field is showing signs of age. Located 116 miles off the north-eastern coast of Scotland, this stalwart of UK hydrocarbons production began life in 1975 and once accounted for 10% of the nation's gas consumption, but this has since declined to around 1-2%. In 2006, as returns continued to slide, Shell began to make plans for the eventual decommissioning of the field.
Shell's infrastructure in the field comprises four production platforms, known as Brent Alpha, Bravo, Charlie and Delta. Each of these will eventually need decommissioning following the cessation of production (CoP), the first of which was the Delta platform in December 2011. This marks the first major milestone in the decommissioning process, following years of careful planning.
"CoP on Brent Delta, on 31 December 2011, marked an important milestone for the Brent decommissioning project, moving us well and truly into the 'execution phase' of decommissioning the platform," explains Austin Hand, Brent decommissioning project director. "It is planned that Brent Alpha and Brent Bravo will continue production at least into next year and Brent Charlie for some time after that."
The decommissioning of each platform will present a slightly different challenge, as each has a unique design; either a concrete-gravity base or steel jacket platform. On top of the four production platforms, each of the wells needs to be plugged and abandoned, and the subsea intrafield third party pipeline infrastructure decommissioned from service. The variety and number of different assets that make up the field will make for a complex decommissioning project.
Oil and Gas UK lists topside removal as the largest expenditure in North Sea decommissioning projects, followed by jacket removal, well plugging and abandonment, project management costs, topside cleaning, conductor removal, pipeline decommissioning, onshore disposal, pipeline cleaning, and, finally, survey and monitoring costs.
Shell has already awarded a number of contracts to cover these myriad tasks. PSN was awarded one of the first major contracts by Shell in July 2010 for work on decommissioning Brent Delta platform's topsides. The contract covers integrity management, safe shutdown, hydrocarbon cleaning, module, process and utility separation, and disconnections.
Aboard Brent Delta, work is already well under way. "We have nearly 160 people working offshore at the moment on Brent Delta," explains decommissioning project manager Derek Allan. "At the same time, as we perform the engineering down and prepare the platform for the major deconstruction campaign of 2013-14 - which in turn will pave the way for the removal programme of 2015 - we still have well plugging and abandonment activities ongoing. This is a major three-and-a-half-year programme to decommission the platform's 40 wells, which will conclude early next year."
Plugging and abandoning the wells is one of the most sensitive projects of the decommissioning process, especially following the Deepwater Horizon oil spill and the resultant intense media scrutiny of BP's attempts to plug the well.
As a result, an extensive Environmental Impact Assessment has been conducted by Shell to ensure the Health and Safety Executive (HSE) aspect of well plugging and abandonment, plus all other aspects of the decommissioning process are fully assessed. A total
of 67 separate reports have been produced on the environmental aspects of the decommissioning project, which study every conceivable HSE impact, ranging from safety risks affecting the local fishing fleet, to waste management and energy use.
Well plugging and permanent abandonment can be achieved in a number of ways, but the most popular method in use today involves cutting and pulling the tubing and casing out of a well, and then sealing it with a concrete barrier.
The challenge is to ensure that the concrete will provide a permanent barrier that seals the well forever, avoiding the need to ever return to the well and reseal it. In the case of Brent Delta's wells, each well will be plugged on a case-by-case basis, but the overarching aim is to provide three down-hole cement barriers per well, each several hundred feet in length.
"Well abandonment is the first milestone on the critical path for the entire project, as installations cannot be decommissioned until the oil and gas in the reservoir have been isolated," explains senior well engineer Iain Scott, who is currently working on the project. Having worked on the Brent field as a drilling engineer and rig manager, he and his colleagues are well placed to work on the abandonment of Brent Delta's wells. "Many of us are very familiar with Brent, and it's useful to have that insight into operating the equipment and the various issues that can be encountered in these wells."
Beneath the waves, the seabed is home to more than just the wells; there is a wealth of other subsea installations that need to be removed as part of the decommissioning project.
"Brent is a large field and the amount of subsea facilities to be decommissioned is significant," confirms subsea facilities team leader John Ironside. "There are 32 pipelines, ranging in length from 0.1km to 36km; four subsea structures weighing more than 100t each; and up to 600 concrete mattresses (used to provide pipeline protection) with a total estimated weight of 2,000t; as well as the seabed debris - such as scaffolding poles lost overboard - which has accumulated during 35 years of operation."
While most of this infrastructure sits within the immediate vicinity of the four production platforms, the pipelines stretch for miles in many directions, adding an extra layer of complexity to their decommissioning. In accordance with regulations, each of the 32 pipelines first needs to be assessed individually, with all feasible decommissioning options considered, in order to decide what course of action is best. It is not as simple as lifting the pipelines up from the seabed and transporting them back to shore; each of the 32 pipelines first needs to be assessed individually to decide the best course of action.
Before any work can be carried out, the pipelines all need to be cleaned to remove any remaining hydrocarbons still present following CoP. Plus, any other contaminants that have built up over three decades of operation will also need to be removed. Decommissioning of the pipelines themselves will take a number of different forms depending on the type of pipe.
For example, untrenched flexible flowlines and umbilical cables, totalling 8.8km in length, will be recovered by reverse reeling, while other flexible products will be left in their existing trenches. Short sections of rigid pipelines will be recovered by cut and lift, but longer sections of untrenched rigid pipeline are more complex to decommission and potential options under assessment range from leave-in-place, through trenching or rockdumping to full removal. There are also hundreds of concrete mattresses, which will need to be removed where possible, depending on the level of degradation they have suffered over the past 35 years.
But in terms of the project as a whole, these few examples are just the tip of the iceberg. With a wealth of experience and a keen eye on HSE requirements, Shell is continuing apace with the Brent field decommissioning project. It won't be long before only a few traces of this oilfield's illustrious history remain.
This article was first published in our sister publication World Expro.